[Feature] Add symbol categories for MetaDefender and VirusTotal

[Feature] Add symbol categories for MetaDefender and VirusTotal

Implemented a category-based symbol system for hash lookup antivirus
scanners (MetaDefender and VirusTotal) to replace dynamic scoring:

- Added 4 symbol categories: CLEAN (-0.5), LOW (2.0), MEDIUM (5.0), HIGH (8.0)
- Replaced full_score_engines with threshold-based categorization (low_category, medium_category)
- Fixed symbol registration in antivirus.lua to use rule instead of config
- Updated cache format to preserve symbol category across requests
- Added backward compatibility for old cache format
- Added symbols registration and metric score assignment
- Updated configuration documentation with examples

The new system provides:
- Clear threat categorization instead of linear interpolation
- Proper symbol weights applied automatically
- Consistent behavior between MetaDefender and VirusTotal
- Cache that preserves symbol categories

Configuration example:
metadefender {
  apikey = "KEY";
  type = "metadefender";
  minimum_engines = 3;
  low_category = 5;
  medium_category = 10;
}